Integrated optical waveguides typically consist of a patterned, light guiding core layer (of refractive index n1) surrounded by a cladding material (of refractive index n2, where n2<n1) and mounted on a mechanically robust substrate. These waveguides generally have flat end faces, often produced by cutting the substrate and waveguide structure with a dicing saw, followed by a polishing step to remove scattering centres. Light propagating along the waveguide is guided within the core by the refractive index difference between core and cladding.
Referring to the drawings, FIGS. 1a and 1b show side and end views of the end face of a typical integrated optical waveguide 10 as known in the art, comprising a substrate 11, a lower cladding layer 12, a light guiding core 13 and an upper cladding layer 14. Depending on the material system, a variety of techniques are available for depositing the lower cladding, core and upper cladding layers. These include flame hydrolysis or chemical vapour deposition (e.g. for glass), molecular beam epitaxy (e.g. for semiconductors) and spin coating (e.g. for polymers). The core layer can be patterned either by photolithography and reactive ion etching (suitable for most materials) or by photolithography and wet etching (e.g. for photo-patternable polymers). The refractive index of the lower 12 and upper 14 cladding layers needs to be less than that of the core 13, so that light is confined within the core. Often, the lower 12 and upper 14 cladding layers have the same refractive index, so that the guided mode is symmetric, although this is not necessary. If the substrate material 11 is transparent and has refractive index lower than the core material 13, the lower cladding 12 may be omitted.
Typically, planar waveguides have a light transmissive elongated core region which is square or rectangular in cross section. The bottom face is conventionally defined as that being adjacent or nearest the substrate. The top face is the face parallel to the bottom face but furthest from the substrate. The sides are those faces which are perpendicular to the substrate.
In this integrated optical waveguide previously described in the art, the core is surrounded by cladding material, either the lower cladding or the upper cladding. However this need not necessarily be the case, and there are some applications where it is advantageous for at least one portion of the core to be free of contact with cladding material on at least one side.
Accordingly, one aspect of the present invention concerns integrated optical waveguides where the upper cladding layer is patterned such that in at least one region, at least one side of the core is free of contact with the upper cladding material.
Patterned upper claddings have been disclosed in U.S. Pat. Nos. 5,850,498 and 6,555,288, for reducing the stress in a waveguide core. In these disclosures, where the patterned upper cladding is described as “conformal”, the upper cladding has a shape substantially congruent with the shape of the core, in other words the core is enclosed (except the bottom, which is in contact with the substrate or lower cladding material) with a thin layer of upper cladding material. This is distinct from the patterned upper cladding of the present invention, where the upper cladding layer is patterned such that in at least one region, at least one side of the core is free of contact with the upper cladding material.
One application where it is advantageous for at least one region of the core to be free of contact with cladding material on at least one side is an integrated optical waveguide with a unitary lens structure. As mentioned above, light is guided along an integrated optical waveguide by the refractive index difference between the core and cladding layers. However when the light exits the core into free space (or air in practice) it immediately diverges. This divergence occurs in two dimensions, parallel and perpendicular to the substrate. If a collimated output beam is desired, some sort of positive (i.e. converging) lens is required. Equivalently, a converging lens is required to focus a collimated beam into an integrated optical waveguide.
One solution is to use a discrete element such as a ball lens or a cylindrical gradient refractive index (GRIN) lens, however such lenses are difficult to handle because of their small size, require precise alignment in two dimensions, and introduce additional interfaces (with inherent reflection losses). It is preferable therefore to integrate the lens structure with the optical waveguide. Many types of integrated lenses have been proposed over the years, including Fresnel lenses (U.S. Pat. Nos. 4,367,916; 4,445,759) and Bragg lenses (U.S. Pat. Nos. 4,262,996; 4,440,468). These lenses provide focussing in one dimension only, in the plane of the lens structure (invariably parallel to the substrate).
Another possibility is to fabricate a GRIN lens at the end face (U.S. Pat. No. 5,719,973). These lenses provide focussing in two dimensions but have cylindrical symmetry and as such are more suited to optical fibres than integrated optical waveguides (which are typically rectangular in shape).
One method for integrating a lens structure with an optical waveguide is to produce a lens-shaped protrusion on the end face of the waveguide. This may be achieved by selectively etching the cladding to leave a protruding core, then heating the waveguide material to its softening point (e.g. with a CO2 laser pulse) so that the angular protrusion collapses into a rounded convex lens shape. Such a structure also provides focussing in two dimensions.
FIGS. 2a, 2b and 2c illustrate a method for fabricating a lens, as known in the art, on the end face of an integrated optical waveguide, as described in U.S. Pat. No. 5,432,877. According to this embodiment, FIG. 2a shows the substrate 11 (e.g. silicon), the lower cladding 12 and upper cladding 14 (both comprising silica doped with boron and phosphorus) and the core 13 (comprising silica doped with boron, phosphorus and germanium). The end face of the waveguide 10 is etched in a buffered hydrofluoric acid solution, which preferentially etches the cladding layers, to leave a protrusion 20 of core material. Finally, the etched waveguide is heated to approximately 1000° C. to soften the core; surface tension then shapes the protruding core to produce a substantially cone-shaped lens 21.
Such chemical etching techniques have been demonstrated for silica glass-based waveguides in U.S. Pat. Nos. 5,432,877 and 6,341,189. They are, however, limited in their applicability, relying on differential etch rates between the cladding (e.g. silica) and the core (e.g. germanium-doped silica). For example, selective chemical etching generally cannot be used for polymer-based integrated optical waveguides. Furthermore, the thermal rounding process can only be used if the core material has a softening point, which excludes non-thermoplastic polymers and crystalline materials such as silicon and other semiconductors. Also, the etching and softening processes must be precisely controlled if the desired lens shape is to be obtained.
A further disadvantage with chemical etching-based techniques is that the lens structures can only be prepared after the optical waveguide circuit chips have been diced (cutting or breaking into individual chips). While many chips can be collected and etched at the same time, this still requires careful handling and extra process steps.
The present invention concerns a method for fabricating an integrated optical waveguide with a unitary lens structure and with a patterned upper cladding that avoids some or all of the abovementioned disadvantages of the prior art. The unitary lens comprises a curved surface through which light is launched into free space. Since this curved surface must have an air interface, any upper cladding must be patterned such that at least this curved surface is free of contact with cladding material.
The unitary lens structures described in the present invention are capable of focussing light in the dimension parallel to the substrate. If focussing in the perpendicular direction is desired, an external lens such as a transverse cylindrical lens could be used. This configuration is superior to other configurations known in the art that require an external ball or GRIN lens, because these require the external lens to be accurately positioned in two dimensions. In contrast, an external transverse cylindrical lens would only need to be accurately positioned in the perpendicular direction. This is especially advantageous in devices with arrays of unitary lens structures, where one transverse cylindrical lens could be used to provide perpendicular focussing for a plurality of array elements.
A second application where it is advantageous for at least one region of the core to be free of contact with cladding material on at least one side is an integrated optical waveguide device where light is directed around curves with small bend radii. This situation frequently arises in the design of integrated optical waveguide devices, since the footprint of a device can be reduced (and therefore more devices fabricated per substrate) by implementing tight bends. Without wishing to be bound by theory, it is well known that introducing a bend into an optical waveguide perturbs the guided modes such that they tend to leak out the side of the bend, resulting in loss of optical power. For large bend radii (i.e. gradual bends) this loss is negligible, but as the bend radius is reduced, there comes a point where the loss becomes unacceptable. For a given bend radius, the loss depends on the refractive index difference between the core and cladding; if this refractive index difference is larger (i.e. the guided modes are more tightly bound), the loss is smaller. Bend-induced loss occurs for both single mode and multimode waveguides. For the multimode case, the higher order modes (which are less strongly guided) have higher bend loss (i.e. tend to be lost first).
Generally, the core-cladding refractive index difference is maximised (and hence bend loss minimised) when the core is surrounded by free space (air in practical terms), i.e. the “cladding” has a refractive index of 1. For integrated optical waveguide devices, bends usually occur in one plane only, parallel to the substrate. Since bend loss only occurs in the plane of the bend (i.e. light leaks out through the side walls), only the side walls need to be “air clad”. In particular, only that side wall on the outside of the bend needs to be air clad. Referring now to FIGS. 1a and 1b: if an integrated optical waveguide device has tight bends, it would be advantageous for the upper cladding 14 to be omitted. The core 13 would then be in contact with cladding material (the lower cladding 12) only on the bottom, which in terms of bend loss is unimportant.
A disadvantage with omitting the upper cladding in planar waveguides is that the mechanical strength of the structure may be insufficient, i.e. the structure cannot be processed and handled using standard techniques. For example, dicing with a high speed saw could dislodge the core from the lower cladding. Also, bare core structures are extremely vulnerable to mechanical damage or to the formation of scattering centres by extraneous dust. For this reason, in integrated waveguide devices with tight bends, it is advantageous to pattern the upper cladding such that upper cladding material is present everywhere except in the regions surrounding the tight bends.
Similarly, in integrated waveguide devices with unitary lens structures, it is advantageous to pattern the upper cladding such that upper cladding material is present everywhere except in the regions surrounding the unitary lens structures.